1. Field of the Invention
The present invention relates to a process for smoothing the surface of a glass substrate. More specifically, the present invention relates to a process for smoothing the surface of a glass substrate for a reflective mask blank used in EUV (Extreme Ultra Violet) lithography (hereinbelow, referred to as “EUV mask blank” in Description). Further more specifically, the present invention relates to a process for smoothing the surface of a glass substrate for an EUV mask blank, which has a concave defect thereon.
The present invention also relates to an EUV mask blank using a substrate for an EUV mask blank, the substrate having a surface smoothed by the smoothing process, and an EUV mask obtainable by patterning the EUV mask blank.
2. Discussion of Background
An EUV mask blank is fabricated by stacking a reflective film for reflecting EUV light and an absorbing layer for absorbing EUV light on a glass substrate in this order. It is most common that the reflective film is formed of a reflective multilayer film, which has Molybdenum (Mo) films as low refractive layers and Silicon (Si) films as high refractive layers alternately stacked therein, thereby having a high light-reflectivity when EUV light is applied thereon.
If a fine irregularity exists on the surface of a substrate used for fabrication of an EUV mask blank, the irregularity has an adverse effect on the reflective film and the absorbing layer, which are formed on the substrate. For example, if a fine irregularity exists on the surface of a substrate, the periodic structure of a reflective multilayer film formed as a reflective film on the substrate is disordered. When a desired pattern of a mask is printed on a photosensitive organic film (a so-called photoresist film) on a Si wafer by using a an exposing system, a portion of the desired pattern may have a lack of pattern or an excessive pattern other than the desired pattern in some cases. The disorder in the periodic structure of a reflective multilayer film, which is caused by an irregularity existing on the substrate, is called a phase defect, which causes a significant problem. It is desired that no irregularities having a size of a certain value or more exist on a substrate.
Non-patent documents 1 and 2 describe requirements relating to a defect in an EUV mask and an EUV mask blank. The requirements for such a defect are significantly severe. Non-patent document 1 has disclosed that a defect having a size of 50 nm or more is not permissible since the presence of such a defect on a substrate disorders the structure in a multilayer film to 5 form an unexpected shape in a pattern projected onto a resist on a Si wafer. Non-patent document 1 has also disclosed that the surface roughness of a substrate needs to be less than 0.15 nm in rms (root-mean-square) in order to prevent the roughness of a line edge from increasing in a pattern projected onto a resist on a Si wafer. Non-patent document 2 has disclosed that it is not permissible that a defect having a size of more than 25 nm exists on a mask blank coated with a reflective multilayer film, which is used in EUV lithography.
Non-patent document 3 describes what size of a defect on a substrate can be printed. Non-patent document 3 has disclosed that it is likely that a phase defect changes a line width of a printed image. A phase defect including a surface bump having a height of 2 nm and a FWHM (full width at half maximum) of 60 nm is a phase defect that is at the border line as to whether the phase defect can be printed or not. This document has also disclosed that a defect having such a size causes an impermissible change of 20% in line width (140 nm on a mask) with respect to a line having a width of 35 nm.
Non-Patent Document 1: SEMI, P. 37-1102 (2002), “Specification for extreme ultraviolet lithography mask substrate”
Non-Patent Document 2: SEMI, P. 38-1102 (2002), “Specification for absorbing film stacks and multilayers on extreme ultraviolet lithography mask blanks”
Non-Patent Document 3: SPIE, vol. 4889, Alan Stivers., et. al., p. 408-417 (2002), “Evaluation of the Capability of a Multibeam Confocal Inspection System for Inspection of EUTVL Mask Blanks”